diff --git a/clock.c b/clock.c
index 5523297..e9449dc 100644
--- a/clock.c
+++ b/clock.c
@@ -692,7 +692,7 @@ struct PortIdentity clock_parent_identity(struct clock *c)
 
 int clock_poll(struct clock *c)
 {
-	int cnt, i, j, k, lost = 0, sde = 0;
+	int cnt, err, i, j, k, lost = 0, sde = 0;
 	enum fsm_event event;
 
 	cnt = poll(c->pollfd, ARRAY_SIZE(c->pollfd), -1);
@@ -710,7 +710,7 @@ int clock_poll(struct clock *c)
 	for (i = 0; i < c->nports; i++) {
 
 		/* Let the ports handle their events. */
-		for (j = 0; j < N_POLLFD; j++) {
+		for (j = err = 0; j < N_POLLFD && !err; j++) {
 			k = N_CLOCK_PFD * i + j;
 			if (c->pollfd[k].revents & (POLLIN|POLLPRI)) {
 				event = port_event(c->port[i], j);
@@ -718,7 +718,7 @@ int clock_poll(struct clock *c)
 					sde = 1;
 				if (EV_ANNOUNCE_RECEIPT_TIMEOUT_EXPIRES == event)
 					lost = 1;
-				port_dispatch(c->port[i], event, 0);
+				err = port_dispatch(c->port[i], event, 0);
 				/* Clear any fault after a little while. */
 				if (PS_FAULTY == port_state(c->port[i])) {
 					clock_fault_timeout(c, i, 1);
diff --git a/port.c b/port.c
index c521c18..681a4dd 100644
--- a/port.c
+++ b/port.c
@@ -1406,7 +1406,7 @@ struct foreign_clock *port_compute_best(struct port *p)
 	return p->best;
 }
 
-void port_dispatch(struct port *p, enum fsm_event event, int mdiff)
+int port_dispatch(struct port *p, enum fsm_event event, int mdiff)
 {
 	enum port_state next;
 
@@ -1432,11 +1432,11 @@ void port_dispatch(struct port *p, enum fsm_event event, int mdiff)
 		next = port_initialize(p) ? PS_FAULTY : PS_LISTENING;
 		port_show_transition(p, next, event);
 		p->state = next;
-		return;
+		return 1;
 	}
 
 	if (next == p->state)
-		return;
+		return 0;
 
 	port_show_transition(p, next, event);
 
@@ -1493,6 +1493,7 @@ void port_dispatch(struct port *p, enum fsm_event event, int mdiff)
 		};
 	}
 	p->state = next;
+	return 0;
 }
 
 enum fsm_event port_event(struct port *p, int fd_index)
diff --git a/port.h b/port.h
index d98f610..74a08f7 100644
--- a/port.h
+++ b/port.h
@@ -68,8 +68,10 @@ struct foreign_clock *port_compute_best(struct port *port);
  * @param port A pointer previously obtained via port_open().
  * @param event One of the @a fsm_event codes.
  * @param mdiff Whether a new master has been selected.
+ * @return Zero if the port's file descriptor array is still valid,
+ *         and non-zero if it has become invalid.
  */
-void port_dispatch(struct port *p, enum fsm_event event, int mdiff);
+int port_dispatch(struct port *p, enum fsm_event event, int mdiff);
 
 /**
  * Generates state machine events based on activity on a port's file